The present invention relates generally to multi-conductor electrical cable and, more particularly, to a multipurpose boot for use with round-to-flat type multi-conductor electrical cables.
Two well known and commonly used types of multi-conductor electrical cables are the round cable and the flat cable.
Round multi-conductor electrical cables, that is, multi-conductor electrical cables in which the conductors are individually insulated from one another and arranged in a group or bundle that is circular in cross-section and in which the conductors are jacketed except for the ends and often shielded, have been used in a variety of applications for several decades. The principal advantage of round cable is that it can be easily bent or angled in most any direction over its entire length, as needed. Other advantages of round cable are that it can be easily and relatively inexpensively shielded and jacketed. The main disadvantage of round cable is that when it is being attached to a connector the conductors have to be connected individually and cannot be mass terminated.
Since the mid 1950's, with the spectacular growth of the computer industry, the flat, or flat ribbon multi-conductor electrical cable, as it sometime is called, has become increasingly popular, especially as an electrical interconnection for digital equipment. In a flat ribbon type of multi-conductor electrical cable the individual conductors are disposed side by side in a row, electrically separated from each other and fixedly held in such an arrangement by some means, such as tape or plastic. Internal and external interconnection of various functions in computers and other instruments have benefited from the ease of use and compactness of flat ribbon cable. In fact, an entire industry has grown around the manufacture of flat ribbon cable and the necessary connectors, preferably, insulation displacement (IDC) connectors, associated therewith. The main advantage of flat ribbon cable is that when it is being attached to a connector the conductors can be mass terminated (i.e. they do not have to be affixed to the connector individually). The main disadvantage of flat cable is that it cannot be very easily or very conveniently bent or angled to accommodate a particular design requirement. Other disadvantages of flat cable are that the conductors cannot be rearranged to suit particular design needs and that the cable is somewhat more dificult and expensive to shield than round cable.
In response to changing standards imposed by the Federal Communications Commission over the past few years with respect to electromagnetic emissions, shielded versions of these cables and the connectors associated therewith are now available. As a result thereof, these flat cables and connectors have dramatically increased in size and expense.
In order to overcome the problems noted above associated with flat ribbon electrical cable, a combination of a round type cable and a flat type cable has been developed over the last several years; that is, a cable which is flat at the ends with flexible conductor alignment and essentially round between the ends where it can be easily bent. The conductors are usually enclosed in a jacket over the portion of the cable between the ends where the cable is round, shielded if desired, and held in the proper side by side alignment at the ends by strips of tape so that the ends can easily be mass terminated. Such a development has also overcome the crowding and air circulation problems heretofore associated with flat ribbon cable. This recent improvement in flat ribbon cable has been referred to as "round-to-flat" multi-conductor electrical cable, since it is round except for the ends and then flat at the ends. Another advantage of this type of cable over flat ribbon cable is that the conductors can be easily positioned at the ends relative to each other to suit a particular application. Electrical tests for crosstalk and impedance uniformity of round-to-flat multi-conductor cables have proven such cables to be electrically superior to both the shielded and unshielded flat ribbon cable of the past. An example of a round-to-flat multi-conductor cable can be found in an article by Darrell Fernald entitled, "An Alternative to Flat Ribbon Cable," Electri-Onices, Dec. 1984.
A number of drawbacks have, however, been associated with the round-to-flat multi-conductor cable. One problem occurs after the cable has been attached to a connector in the portion of the cable where the configuration changes from round to flat; that is, between the ends where the cable is flat and the jacket where the cable configuration is round.
More specifically, in the portion between the connector where the conductors are arranged flat and the jacket where the conductors are round (and hereinafter referred to as the transition area) the conductors are in a loose bundle, are not jacketed, and are not held in any particular alignment. Consequently, any pulling, accidental or intentional, of one or more of the conductors in this transition area may cause these conductors so strained to tear away from the connector to which they are attached. Since the conductors are not attached to each other in the transition area any strain applied to any one particular conductor will not be spread out over the other conductors. Additionally, the conductors in this area are vulnerable to abrasion and other undesirable effects of the surrounding environment. Even further is the concern associated with the positioning of the conductors over this area to provide a desired angular orientation of the cable with respect to the connector to which it is attached or other associated electronic equipment. For example, it has proven difficult to easily and compactly make a ninety degree bend over this area in any direction since loose wires will tend to spread out and end up in a multitude of directions.
Another disadvantage of round-to-flat cable is that because the conductors in the transition area are loose they have a tendency to snag or catch on sharp edges, such as may be present on printed circuit boards, as the cable is being slid into a chassis or housing to its intended position.
In the past, an attempt has been made to solve some of the problems associated with round-to-flat cables over the transition area by placing a piece of shrink tubing, such as a heat shrinkable plastic, over the uncovered conductor portions and then shrinking the tubing (by heat) down to a tight fit. Although this arrangement does provide physical protection of the conductors against electrical shorts where they are not jacketed, it does not provide any strain relief against the individual conductors and in many instances distorts the alignment of wires.
Another technique that has been employed in the past for providing protection and strain relief of the conductors in the transition area has involved using an attachment referred to in the art as a back shell. This attachment, which is made of either metal or a rigid plastic, comprises two mating shell shaped sections which when mounted on the cable fit over the end of the jacketing in the round portion, fit over the conductors in the transition area and fit around and clamp over the connector. Advantages of the back shell are that it does protect the wires in the transition area and does indirectly connect the jacket in the round portion to the connector so as to provide strain relief of the individual wires. The disadvantages of the back shell are that it is cumbersome, bulky, not flexible, expensive to fabricate and is limited in use to only the particular sized and shaped connector for which it is constructed. As can be appreciated there are a variety of different sized and shaped connectors.
As is apparent, a great need exists to provide an effective means of alleviating the above-mentioned problems associated with round-to-flat multi-conductor electrical cable.